Mysterious mass oyster die-off solved

Oysters clinging to the shoreline at China Camp State Park in San Francisco Bay, before the mass die-off in 2011.
(c) Brian Cheng / UC Davis

Wild oysters crowd the shoreline at China Camp Beach in San Francisco Bay in 2011. Nearly all of them perished due to an atmospheric river event in 2011.
(c) Brian Cheng / UC Davis

In 2011, there was a mass die-off of wild Olympia oysters in north San
Francisco Bay, the cause of which remained a mystery for a long time.
But not anymore.

Researchers from University of California, Davis have discovered the reason for the mass die-off: atmospheric rivers.

In March 2011, a series of atmospheric rivers found their way to
California, bringing with them intense rainfall. This led to a massive
discharge of freshwater into the San Francisco Bay, which experienced a
drastic drop in salinity within a few days to a level that it was too
low for the oysters to survive.

These atmospheric rivers comprise
narrow stretches of concentrated moisture – several thousand kilometres
long – in the lower atmosphere. They have been the cause of heavy rain
in the region.

The research team, led by Brian Cheng, reconstructed the 2011 incident,
and published their findings in the recent issue of Proceedings of the
Royal Society B.

Atmospheric rivers transport huge amounts of moisture around the world.
If they are at land, severe precipitation such as rain or snow can
result, leading to floods. According to the researchers, the ten most
severe floods in Great Britain since 1970 have been associated with
atmospheric rivers.

The research team had been studying a population of Olympia oysters
(Ostrea lurida) in north San Francisco Bay from October 2009 to July
2011.

Every three months, they would take note of the population size.

Then came March 2011, which brought along the rain caused by
atmospheric rivers. The salt content in the bay dropped to a great
extent and, for eight consecutive days, remained below the level which
the researchers had concluded was the threshold for low salinity for
the oysters.

By July 2011, almost all the oysters had died.

By November 2013, the population of oysters in the region had
recovered. However, according to Cheng, “These new oysters are smaller
and less fertile, and that may have consequences for restoring oysters
in San Francisco Bay.”